reply to your answer

Question
QUESTION: so you are saying for each 1 inch oranda, black moor i would require 30 gallons, or a tank approx 24 x 24 x 145 inches?

my question :-

i have recently set up a tank for my son, and he has said he would like some oranda and black moors. can you suggest any other fish that would be a good mix with these type. the tank is 30l - 6 gallons.

Because Goldfish need at least 30 gallons. Anything less and they won't live more than a week.

ANSWER: No, 30 gallons is more like 36x12. But yes, goldfish grow over a foot long, and in fact it is illegal to keep them in tanks under 30 gallons in some european countries.

I already told you, 6 gallons is enough maybe for some shrimp or a betta, if you want your son to like animals, you shouldn't be encouraging him to do such a form of animal cruelty that is actually illegal in some places.

---------- FOLLOW-UP ----------

QUESTION: I appologies for any offence you may have taken, i am only explaining what i have been told by the shop where i got the tank, which is 24 x 12 x 12, they also said that fan tails and black moor generally only grow to 5 inches. i appologise again for my mis-information.

Answer
No problem.

Pet stores will tell you anything to make a sale. Opposed to people online who only benefit if their information is correct and helpful.

Likely the only reason they say they grow to 5" is

1) They want to sell it
2) All their goldfish's have died before they reached full length
3) Mal-nutrition and poor water quality has stunted their growth

Sure, you commonly see 6-8" goldfish, but the real maximum size is around a foot. And yes, I do have goldfish this large. You should be planning for the fish's max size, not hoping it stays smaller, correct?

Either way, lets just assume that it does stay small, around 6". How long is a 6g? 15"? 12"?

Doesn't seem like much. The very, very, very general rule is to not keep a fish in a tank that isn't at least 10x its body length. That is very general though, so don't rely on it too heavily. But it does go to show that only 2 or 3x is not nearly enough.